1. Field of the Invention
This invention relates to an aircraft sliding door system, particularly advantageous for use in helicopters.
2. Description of the Related Art
A variety of door constructions have been utilized in helicopters. These include hinged doors and sliding doors. Common sliding doors such as those found on the UH-60 BLACK HAWK helicopter by Sikorsky Aircraft Corporation are mounted entirely external to the aircraft fuselage and fit over an opening in the fuselage when in a closed position. Such a door slides between open and closed positions on tracks. These tracks are located above and below the opening, are substantially parallel to the aircraft skin, and extend longitudinally beyond one side of the opening (e.g., beyond the rear side of the opening). In such a surface mount configuration, the outer surface of the sliding door still protrudes beyond the skin of the helicopter when in the closed position. Such a surface mount configuration has advantages of relatively light weight, ease of manufacture, and ease of maintenance. It has disadvantages as well. The projecting door interrupts airflow over the skin of the helicopter and thus creates aerodynamic drag. The airflow interruption also creates noise. The impact of these phenomena increases as the helicopter's speed increases. Additionally, certain uses of the helicopter may be more sensitive to these phenomena (e.g., use as a passenger helicopter vs. use as a military cargo helicopter). Such a surface mount configuration, with linear motion of the door parallel or nearly parallel to the fuselage, makes it difficult to provide an effective seal between the door and fuselage. The linear motion also presents radically different sealing conditions along the sides of the opening compared to along the top and bottom of the opening.
Another sliding door construction is used on the EH101 helicopter made by British Aerospace. This construction provides a flush closing door and features a pair of tracks extending longitudinally along the outside of the fuselage above and below the door opening. The door is carried along the tracks via pairs of upper and lower arms. The pairs of upper and lower arms respectively link the door to the upper and lower tracks. At one of its ends, each arm is pivotally connected to the door. At the other end, each arm is pivotally connected to a single rider on the associated track. When viewed from above, the door, arms, and rider form a parallelogram mechanism. In its closed condition, the door is recessed flush within the opening. The door is pulled outward to open, thus causing rotation of the arms relative to the door and riders in an articulation of the parallelogram mechanism. The door is pulled clear of the fuselage in this manner, whereupon the door, arms, and riders as a unit may be slid along the tracks to move the door into an open position wherein it does not block the opening. Closing of the door is performed in a reverse of this process.
As this type of flush-closing door approaches its closed position, movement of the door is substantially inward (i.e., nearly perpendicular to the opening). This allows for a smooth engagement of a seal circumscribing the opening (i.e., contact between the door and seal is close to straight-on, substantially compressive, and with little shear). However, providing a sufficient amount of compressive force to that seal is difficult. For a desired degree of sealing using a bulb-type seal, an advantageous normal closing force of about one pound per linear inch of seal is believed to be required. Where the door size is about five to six feet in height and two to three feet in width, between about fourteen and eighteen linear feet of seal is required. At one pound of closing force per linear inch, this would require someone pulling the door inward to apply a closing force of about 168-196 pounds. As such an amount of force is difficult to provide, a weaker, more flexible, seal material and/or configuration (i.e., one requiring a smaller closing force) is provided. Such a weak seal may not effectively resist water, air, and sound infiltration to the degrees which may be desirable. There is thus a need for an improved sliding door construction for an aircraft. Such an improved construction will advantageously provide both a flush-closing door and a strong seal under a wide variety of conditions without the need for an excessive applied closing force.